This invention relates generally to electrical switches and more particularly to switches using spring disc elements which move between opposite convex and concave configurations and which are actuated upon the occurrence of selected conditions such as pressure. This application includes subject matter also contained in copending, coassigned application Ser. No. 114,487 filed 10/28/87 which is a continuation-in-part of Ser. No. 946,438, filed 12/23/86, now abandoned.
Conventional condition responsive switches have a contact arm movable between first and second switch positions prebiased to one switch position and have a dished snap acting disc element movable between opposite convex and concave configurations for moving the contact arm between switch positions in response to the occurrence of selected temperature or pressure conditions. Such switches are intended to perform selected control functions in response to the occurrence of the selected temperature or pressure conditions in a zone to be monitored. An example of a switch of this type is shown and described in U.S. Pat. No. 4,581,509 which issued to the assignee of the present invention.
This type of switch has become widely used, among other applications, in automotive environments such as in air conditioning refrigeration compressor systems. For example there is a need in such a system to provide a switch to protect the system from excessive high pressure. Additionally, there is a need to provide a switch to protect the system from a loss of freon and lubricant charge and resulting compressor damage. Both of these switches are connected to operate the compressor clutch either directly or through a computer control system and are typically mounted in the compressor housing and communicate with the high pressure side of the system. The high pressure protection device typically opens on pressure increase to about 430 psi while the high side low pressure switch typically closes on pressure increase to 50 psi.
In copending application Ser. No. 114,487, referenced supra, an improved switch is shown comprising a normally open electric switch mounted adjacent first and second vertically aligned discs adapted to move from one dished configuration to an opposite dished configuration upon the occurrence of selected conditions. The first disc has a normally concave surface configuration facing the switch and has a centrally disposed aperture through which a motion transfer pin extends between a movable contact arm of the electric switch and the second disc having a normally convex surface configuration facing toward the switch. The second disc is mounted in a pressure-force converter which is adapted to move the second disc toward a reaction surface. The first disc is adapted to invert its curvature upon being exposed to increasing pressures of a selected first level or above, and the second disc is adapted to invert its curvature upon being exposed to increasing pressures of a selected second, high level or above. At pressures below the first level, the first disc prevents actuation of the switch and at pressures above the second level the second disc allows deactuation of the switch.
The switch described above is very effective in combining functions in a single housing thereby conserving space and material, however, in using a diaphragm or membrane as a means of communicating fluid pressure to the switch mechanism the added displacement required of the membrane produces stresses which may reduce the number of cycles to which the membrane can be subjected during its useful life. Further, the use of the pressure converter described in the copending application may effect the specific amplification factors required for each disc making their selection with regard to factors such as yield, reliability and fatigue life more critical. That is, one disc has a low pressure amplification factor required for its actuation while the other has a high pressure amplification factor required for its actuation.
It is therefore an object of the present invention to provide apparatus which will extend the useful life of the membrane used as a means of communicating fluid pressure to a switch mechanism, particularly in a device having more than a single function. Another object is the provision of apparatus which reduced the criticality of the selection of discs used to actuate the switch.
Briefly, in accordance with the invention a fluid pressure switch has a flexible membrane for communicating fluid pressure to a switch mechanism through a movable pressure-force converter. The converter comprises at least two portions movable relative to one another in engagement with the membrane and stop means to limit movement of one portion relative to the other portion so that both first and second portions are movable in response to selected pressure levels until the stop means limits movement of the one portion while the other portion continues to be movable at increased pressure levels.
According to a feature of the invention when used with a dual function pressure switch having first and second discs movable between convex and concave configurations, the first and second portions of the pressure converter form a first area and move together in response to increases in pressure to effect the actuation of the first disc and the second portion of the pressure converter forms a smaller area and is movable in response to further increases in pressure to effect the actuation of the second disc.